During a large part of HW (3.x patches) tank damage was actually based on both STR and VIT (each contributing 0.45 AP)
This was changed back to having tank damage based on STR only in 4.0. The reasoning given was something about this making it easier to balance damage output and health.

Since they have already tried this once and then gone back to the old system, I doubt the devs will change the damage calculation for tanks again any time soon.